l62 PRACTICAL BACTERIOLOGY. 



would run into cue another, and thus render a correct 

 statement of their number and varieties impossible. 

 There is no need of hesitation in the matter, for if so 

 many bacteria are presenb as to render an earlier ex- 

 amination necessary, it may be taken for granted that 

 the species which would develop later on are, com- 

 paratively speaking, of only small importance. 



The real examination is founded upon two consider- 

 ations. First, note must be taken of the number of 

 the colonies which have developed; and secondly, the 

 number and character of the species present must bo 

 most accurately observed. 



If there are not many colonies, they may easily be 

 counted ; if, however, there are a great many, simple 

 counting could hardly be relied upon, and in any case 

 would take up a great deal of time. It there are 

 several thousands of colonies, one more or less does 

 not make much difference, and they are reckoned up 

 only in round numbers. There is a simple apparatus 

 which very much simplifies this matter of counting 

 the colonies. This is illustrated in Fig. 9. It consists 

 of a wooden stand upon which a slate is fixed ; over 

 this rests a glass plate, which is divided into square 

 centimetres, and which is supported on little blocks. 

 Most of the glass dishes are about 60 square centi- 

 metres in area, that is to say about 6 centimetres in 

 diameter. The culture plate is then placed upon the 

 slate, and is covered by the glass plate. The colonies 



